Orthogonal codes based Code Division Multiplexing (CDM) techniques are widely applied in the technical field of wireless communication. The most classic CDM technique is to expand different signals by using different orthogonal sequences, and superpose them so as to eliminate interferences between the superposed signals by means of the orthogonal property among different sequences. Because of this advantage, the CDM techniques are widely applied for multiplexing different signals in the wireless communication system.
FIG. 1(A) to FIG. 1(D) are diagrams showing the principle of the CDM multiplexing using four-dimensional Walsh codes. As shown in FIG. 1(A), the code words used in CDM are orthogonal to each other, which means correlations between the different code words are zero. As shown in FIG. 1(B), in the CDM multiplexing, different signals S1, S2, S3, S4 correspond to different code words respectively, and the different signals are respectively multiplied by chips in the corresponding code words. The result of the multiplication produces expansions of signals. The expansions produced by the different signals are superposed to form the multiplexed signals W, X, Y, Z. As shown in FIG. 1(C), the multiplexed signals W, X, Y, Z are transmitted on a communication channel. The expansions of signals by CDM may be performed either on the time domain or on the frequency domain. As shown in FIG. 1(D), in the CDM de-multiplexing, the signals after the CDM expansion are correlated with the corresponding code words to recover the original signals S1, S2, S3, S4.
In the CDM multiplexing using orthogonal codes, the orthogonality among the different orthogonal code words is the most essential characteristic of the conventional orthogonal CDM. In the wireless communication, the most widely used orthogonal code is Walsh code, and the length of such code can be 2, 4, 8, 16 . . . (a power of 2). The different orthogonal code words may form an orthogonal matrix.
FIG. 2 is a schematic diagram showing that different base stations transmit multiple data streams to a mobile terminal in a wireless communication system.
As shown in FIG. 2, adjacent base stations 201 and 202 may include multiple antennas respectively, and transmit multiple data streams to a mobile terminal 203 in a way of spatial multiplexing respectively. The data streams may be divided into multiple layers, for example, each data stream may include two or more layers of data stream. Here, it is shown that each data stream includes a first layer of data stream and a second layer of data stream respectively.
FIG. 3 is a diagram showing an example of a resource block constituting a data stream transmitted to a mobile terminal from a base station in a wireless communication system.
In FIG. 3, one resource block (RB) constituting a data stream is shown. The horizontal axis of the resource block represents time, while the vertical axis represents frequency bandwidth. The horizontal axis is divided into 14 segments, each of which forms one OFDM symbol along the vertical axis beginning at the horizontal axis. The vertical axis is divided into 12 segments, each of which is one sub-carrier along the horizontal axis beginning at the vertical axis. Each of small squares in the resource block represents one resource unit. All of 12×14 resource units in the resource block constitute one sub-frame on the horizontal axis. The first three columns of the resource units in the resource block constitute a control region for transmitting control data. Other resource units without grid lines are used to transfer data signals. In the same base station including multiple antennas, for example, in the base station 201, the multiple data streams may be transmitted to the mobile terminal 203 in a way of spatial multiplexing. The multiple data streams are located at different layers respectively, and each layer of data streams of the resource block may use the same time and frequency resources. For example, the multiple antennas of the base station 201 may transmit two layers of data streams, that is, a first layer of data streams and a second layer of data streams, to the mobile terminal 203 through spatial modulation, and the corresponding resource blocks in each layer of data streams may be located at the same time and frequency resources, that is, at the same time and frequency but using different pre-coding manners.
Resource units 301 represented by grid lines are used to transmit demodulation reference signals (DMRS) of a dedicated channel specific to a cell, the demodulation reference signals are used to demodulate data signals transferred in the resource block in a mobile terminal. Here, each resource block includes multiple demodulation reference signals which are distributed at predetermined time and frequency positions. In order to correctly demodulate the data in the multiple layers superposed on the time and the frequency, the LTE-Advanced provides the demodulation reference signals (DMRS) which are orthogonal with each other for the superposed data layers.
FIG. 4 shows an example that the different layers of demodulation reference signals are multiplexed by using an orthogonal matrix.
FIG. 4 is an instance in the LTE-A Release-9 standard. In FIG. 4, Walsh codes with the code length of 2 such as [1, 1] and [1, −1] are used to multiplex two layers of demodulation reference signals orthogonal with each other. Specifically, the Walsh code [1, 1] is multiplied by respective demodulation reference signals in the first layer of the resource block, and the second code [1, −1] of the Walsh matrix is multiplied by respective demodulation reference signals in the second layer of the resource block.
FIG. 5 shows a sectional diagram of a resource block after Walsh codes with the code length of 2 such as [1, 1] and [1, −1] are used to multiplex two layers of demodulation reference signals.
The result after multiplexing two layers of orthogonal demodulation reference signals on one resource block is as shown in FIG. 5. For the sake of clarity, FIG. 5 shows only a part of the resource block. In FIG. 5, it is assumed that the pre-coding factor for demodulation reference signals in the first layer of the resource block is A, and the pre-coding factor for demodulation reference signals in the second layer of the resource block is B. In the adjacent two OFDM symbols placed with the demodulation reference signals, one always has a value of A+B, the other always has a value of A-B. When A=B, one symbol always has a peak value of (A+B), and the other symbol has always a value of zero. However, in order to ensure the usage efficiency of a Power Amplifier (PA) in a base station, power fluctuation on the time (that is, between the different OFDM symbols) of emission power is required to be as little as possible. If the above mapping manner from the orthogonal multiplexing codes of the demodulation reference signals to the resource block is employed, when A=B (as shown in FIG. 5), peak values and zero values alternately appear in the OFDM symbols containing the demodulation reference signals, which will cause the power fluctuation between the different OFDM symbols increasing. In order to address the problem, a mapping manner as shown in FIG. 6 is actually employed in the LTE-Advanced Release-9.
FIG. 6 shows an actual mapping manner of the code division multiplexing based on the orthogonal codes in the LTE-A Release-9.
In FIG. 6, RB1 and RB2 are two resource blocks adjacent on the frequency domain. The characteristic of such a mapping manner is that, for the demodulation reference signals multiplexed with the code word [1, −1], mappings thereof on the resource blocks are alternately reverse on different sub-carriers. The result of such a mapping manner is shown in FIG. 7.
FIG. 7 shows a sectional diagram of resource blocks after the Walsh codes with the code length of 2 such as [1, 1] and [1, −1] are used to multiplex the two layers of demodulation reference signals in the LTE-A Release-9.
It can be easily seen, by comparing FIG. 5 with FIG. 7, that the peak value (A+B) and zero value appear alternately on the different OFDM symbols when A=B, which reduces the impact of power fluctuation on the power amplifier.
However, when the number of layers of demodulation reference signals for code division multiplexing is multiple, the case that the peak values and the zero values cannot be distributed evenly as shown in FIG. 4 still exists.